Trace gas analyzer



April 23,

Samp/e aan 44 W. C. PFEFFERLE TRACE GAS ANALYZER Filed Dec. 50. 1959 0fGas 7b Be Haifa/pt 60k/mf? United States Patent O 3,087,112 TRACE GASANALYZER William C. Pfeferele, Middletown, NJ., assigner, by

mesne assignments, to Engelhard Industries, Inc., Newark, NJ., acorporation of Delaware Filed Dec. 30, 1959, Ser. No. 862,813 16 Claims.(Cl. 324-33) This invention relates to gas analyzers, and more par--ticularly to analyzers for detecting traces of gases in proportionssigniiicantly less than one part per million.

In the field of gas chromatography, gases are customarily analyzed bythe application of a sample to an elution or chromatographic column; thecomponent gases in the sample are separated by the different speeds ofmigration through the column; and the amount of gas of each type can bedetected during successive time intervals at the output of the column.Up until recently, however, the sensitivity of analyzers employing theprinciples of gas chromatography has been relatively low and generallyhas been considerably poorer than one part per million. Furthermore,some of the higher sensitivity devices are restricted in application.

Accordingly, a principal object of the present invention is to improvethe sensitivity of gas analyzers, without sacrificing versatility.

iFrom a comprehensive aspect, one illustrative embodiment of theinvention includes two successive chromatographic columns, a source ofcarrier gas such as hydrogen or helium having less than one part permillion of extraneous gases of significantly lower ionization potentialthan that of the carrier gas, and a secondary ionization detector havinga sensitivity considerably greater than one part per million. 'Ihecarrier gas may be hydrogen or helium purified by diffusion throughpalladium or quartz, respectively.

The secondary ionization detector in the illustrative embodiment of theinvention may be an electronic tube which includes at least fourelectrodes. These electrodes include a cathode, rst and second grids,and a plate. The potential between the cathode and the iirst of the twogrids, the accelerating grid, is posit-ive and has a value significantlygreater than the ionization potential of the gas which is beingdetected, and a value which is insuicient to ionize the carrier gas.Thus, for example, when oxygen having an ionization potential of 12.5volts is being detected, and when hydrogen having an ionizationpotential of 15.6 volts is used for the carrier gas, the potentialbetween the cathode and the accelerating grid may Irbe about 14 volts,slightly less than the ionization potential of hydrogen. rIhe second orshielding grid which is provided has a negative potential with respectto the first grid. This potential may, for example, be approximately 30volts negative with respect to the first grid. A negative voltagesufficient to ionize the carrier gas may be applied to the plate of thetube.

-In operation, the electrons from the cathode are accelerated toward thefirs-t grid and ionize the oxygen in the region between the cathode andthe first grid. Positive ions are then drawn toward the shielding gridand the plate which are negatively charged. The high negative potentialof up to about 300 volts between the screen grid and the plate permitssecondary ionization of the carrier gas, thus providing considerableamplification within the tube. The use of the shielding grid isdesirable to avoid penetration of the eld provided by the high negativevoltage at the plate into the region between the cathode and the firstgrid.

In accordance with a feature of the invention, a gas to be detected iscarried to a tetrode secondary ionization electron tube detector by acarrier gas having a higher ionizai-ton potential than the gas to bedetected.

ice

In accordance With additional features of the invention, the carrier gasdescribed in the preceding paragraph may be hydrogen or helium purifiedby diffusion through palladium or quartz, respectively, and the carriergas and the sample to be analyzed are passed through one or morechromatographic columns to separate the gas to be quantitativelyanalyzed from other gases.

Other objects, features and advantages of the invention will becomeapparent from a consideration of the following detailed description andfrom the drawing, in which,

FIG. 1 is a block diagram of a gas analyzing system in accordance withthe present invention;

FIG. 2 is a schematic circuit diagram of the secondary ionization gasdetector in accordance with the invention which may be utilized in thesystem of FIG. 1; and

FIG. 3 is a detailed Iblock diagram of a portion of the system of FIG.1.

With reference to the block diagram of FIG. l, it will be assumed thatit is desired to measure the concentration -of a gas such as oxygen in agas stream indicated -by block 12. The principal components of thesystem include the secondary ionization detector 14 and its associatedrecorder 16, the cycle timer 18, and a source of .pure carrier gas whichincludes the cylinder of hydrogen 2) and the palladium diffusionpurifier 22. 'Ihe hydrogen purifier may, for example, be of the formshown in U.S. Patent No. 2,911,057, granted November 3, 1959 to R. B.Green et al. In addition to conventional shutoit valves (not shown), thesystem includes lthe six port sampling valve 24 and two four port valves26 and 28. The columns 30 and 32, which are provided for separating4different types of gases, also form an important part of the system.Suitable pressure regulators 34 and 36 a-re provided in the input linesfor the purified carrier gas.

Before considering the mode of operation of the system of the presentinvention in detai-l, reference is made to the following referencematerials dealing with the subject of gas chromatography. These includeGas Chromatography edited by Vincent I. Coates, Henry I. Noebels, andIrving S. Fagerson, Academic Press, Inc., New York, 19'58; Vapour PhaseChromatography edited by D. H. Desty, London, Butterworths ScientificPublications, 1957; An Ionization Gauge Detector for Gas Chromatographyby S. A. Ryce and W. A. Bryce, pp. 1293 to 1297, Canadian Journal ofChemistry, vol. 35, 1957.

The sample Valve assembly 24 ris connected to the gas stream 12 to bemonitored by the tubes 3S. The input tube 40 supplies carrier gas, andthe output tube 42 from valve 24 .is connected by valve 26 to the firstcolumn 30. In addition, :the valve assembly 24 has a local sampling loop44 associated with it. In the position sholwn in FIG. 1, the gas fromthe source 12 circulates through the sample loop 44. Upon rotation ofthe core of the six port valve 24 by 1/6 of a turn in either direction,the sample loop 44 is connected in series with tubes 40l and 42, butdisconnected from the source 12. This operation introduces 'a sample ofaccurately determined volume into the first column 30 which volume isthat defined within the sample loop.

The use of elution columns for the separation of gases is well known inthe field of gas chromatography, and a number of different forms ofcolumns are described in the texts cited above. In the present case, itis desirable to make a preliminary separa-tion of gases in the firstcolumn 30 and a final separation of gases in the second column 32. Thefirst column 30 could therefore be a partition column, whereas thesecond column 32 could be an adsorption type column. Both types ofcolumns are disclosed in the texts cited above. The partition columnmight characteristically include liquid such as 3 dibutylphthalate orsilicone oil `on -a solid such las granular fire brick. The adsorptioncolumn 32 could suitably be of the type disclosed on pages 247 et seq.of the Desty text cited above. y

The system as disclosed in FIG. l is particularly applicalble to theanalysis of xed gases such -as oxygen, yfor specic example. As mentionedabove, separation is obtained in chromatographic columns by thedifferent times required for gases lof different types to migratethrough the column. Two columns are employed in order to preventpoisoning of the adsorption column 32 by gases which are not reversiblyadsorbed. The partition column 3% permits the rapid migration of gasessuch as oxygen, argon, nitrogen, etc., while other heavier gases havinghigher boiling points, such as propane, traverse the partition columnk30 at a slower rate. The heavier gases migrate reversibly through thepartition column and' ymay therefore be vented from column 30 in thecourse of a reverse ilushing operation, while separation of nitrogen,oxygen and similar gases is occurring in the adsorption column 32. It`may be noted that Vthese last mentioned gases are not separated 'in timeto any considerable extent as they diffuse through the partition column.The use of the two columns therefore minimizes aging of the adsorptioncolumn by avoiding contamination or poisoning by the heavier gases. Incertain cases in which the quantity of oxygen or ano-ther gas is to lbedetermined in gas streams having a known composition analyzed.Characteristically, both columns ycould ibe approximately ve feet inlength, and -the time of diffusion for oxygen about two minutes. Passageof the oxygen through the column 30 takesplace in the directionindicated by the dashed line arrows 46 and 48, valves 26 and 28 being ina position one quarter turn from the position 'shown in FIG. 1. Thus theoxygen passes through valve 26, the partition column 30, and valve 28into the -adsorption column 32. Following the passage of the oxygen fromcolumn 30, the valves 26 and 2.8 are turned to the position shown inFIG. 1 and the partition column is reverse flushed, with the gasproceeding through the partition column in the direction indicated bythe solid arrows 47 and 49 and nally out t-he vent connected to valve28. Pressure is maintained on the column 32 by the source of carrier gasprovided through the pressure regulator 36 and line 50.

The chromatographic columns 30` and 32 are preferably operated atatmospheric pressure or slightly above. In this manner errors whichcould result from slight leaks in the system are avoided. To accommodatethe atmospheric pressure in the columns and the low pressure required inthe detector 14, a pressure reducer 51 is pro-- vided. This reducercould be in the form of a suitable constriction in the flow line, aAbleed-off vent, or could include both of these structures.

The secondary ionization detector 414. includes an electronic tube andits associated circuitry as described in detail l'below in connectionwith FIG. 2. Electrical output signals from the detector 14 iaresupplied on electrical lead 52 to the recorder 16. The output gas line'54 from Vthe detect-or 14 leads to a pump (not shown).

The mode of operation of the system of FIG. 1 is controlled by aconventional cycle timer 18 which may be of Iany standard form. Asynchronous clock motor with adjustable electrical contacts may suitablybe employed. The cycle timer 118 controls the position of the six vportvalve '24 as indicated by the dashed line 56, the position of valves 26and 28 by a common mechanical connection as indicated by dashed lines58, and the operative periods of the recorder .16 as indicatedschematically by the dashed line 60. Conventional electromechanicaloperating devices maybe employed. The sequence of operation of the cycletimer 18 is indicated by the following steps:

Step l The six port sample valve 24 is 4tur-ned to the sample position,one-sixth turn from the position shown in FIG. 1.

Step 2 Step No. 2 occurs following a time interval sufficient for thegas being analyzed to pass through partition column llt, and intoadsorption column 32. At -this time valves 26 and 28 are rotated toreverse gas flow in partition' column 30 and to vent gases from thepartition column (the position shown in FIG. 1). Pressure is maintainedon the adsorption column 32 from line 5d.

Step 3 Immediately following Step 2, the six port sample -valve 24is'returned to its original position as shown in FIG. l of the drawings.

Step 4 For a time interval bracketing the time at which the ygas inquestion leaves the adsorption column 32, the

'recorder 16 is enabled and provides a quantitative indication oftheamount of the gas in question in the sample.

Step 5 partition column 30 by reverse flushing.

Steps 6 et seq.

The cycle set forth above is now repeated. In addition tothe use ofhydrogen puried by diffusion through palladium as discussed above,diffusion `purified helium may also be used as a carrier gas. In

the case of helium, however, diffusion through quartz is preferred todiffusion through palladium. Argon,

'nitrogen or any other suitable gas or combination of gases may beemployed whenV purified to the required extent (signilicantly lless ofmore readily ionized impurities than one part per million). Theionization potentials for these various gases are 24.6 volts for helium,15.7 volts for argon, and 15.6 volts for hydrogen. It is, of course,necessary to select a carrier gas having an ionization potential whichis greater than that of the gas or gases which are being analyzed.

Considering the matter of reversing the ow of carrier gas through thepartition column 30, this is done to eliminate portions of the originalsample which may still be within the partition column. 'The flow of gasthrough ya partition column is normally a reversible process.Accordingly, the time for 'black flushing need only be approximatelyequal to the time of ow in the forward direction. A slight additionaltime period is secondary ionization detector 14 of FIG. 1 is set forthin some detail. In FIG. 2, the electron tube may be of the general typeknown commercially as a 6CB6A, provided with input and output connectingtubes. However, unlike Ithe commercial tube, the heater and the heatedcathode must be resistant to oxidation. Accordingly, the cathode ispreferably of gold or silver and may also be of iridium, and platinumIalloyed with 10 to 40 percent of rhodium may be used for the heaterfilament. Any other suitable materialswhich are sutlciently resistant tooxidation may be used.

The tube 60 in FIG. 2 preferably includes an indirectly heated cathode62, an laccelerating gridv 64, a

shielding grid 66, and a plate 68. A carrier gas, such. as hydrogen, andthe gas, such as oxygen, which is being detected, are supplied throughthe inlet and outlet tubes 70 and 72.

The detector, in accordance with an important aspect of the presentinvention, is based on the principle of selective ionization of tracecomponents in a gaseous carrier, wherein the carrie-r has an ionizationpotential which is higher than the gas which is to be detected. Thepotential between the cathode 62 land the grid 64 is positive so thatelectrons from the cathode 62 are accelerated toward grid 64. Thepotential is adjusted to a level such that the carrier gas will not beionized to any appreciable extent but that a portion of the gas which isbeing detected is ionized. Thus, for example, when oxygen having anionization potential of 12.5 volts is carried through tube 60 by acarrier gas such as hydrogen having an ionization potential of `about15.6 volts, the potential from cathode to grid may be about 14a' volts.As this value is just below the ionization potential for hydrogen, theelectrons from cathode 62 will not attain the energy required to ionizehydrogen, and no appreciable amount of hydrogen will be ionized. As theoxygen reaches the ionization detection tube 60, however, appreciableionization of the oxygen takes place. The positively charged oxygen ionsare drawn toward the shielding grid 66 which is at a negative potentialwith respect to both the accelerating grid 64 and the cathode 62. A highnegative potential of up to about 300 volts is applied between theshielding grid 66 and the plate 68. As the positive oxygen ions areaccelerated through shielding grid 66 toward the plate 68, the carriergas is also ionized and considerable amplification of the signal isobtained. This secondary ionization is linear or a monotonie function ofthe primary ionization and produces a device having much greatersensitivity than is possible with the ionization detectors such as thosediscussed in the references cited above.

As noted above, the negative voltage applied to the plate is of theorder of magnitude of up to 300 volts. Higher voltages may also beemployed up to the breakdown point of the tube. This breakdown pointwill depend among other things on the geometry of the tube,

the types of gases which are present, 'and the pressure in the tube.When the plate voltage is adjusted to a value somewhat less than thebreakdown point of the tube, maximum amplification and sensitivity ofthe apparatus is secured.

The use of the shielding grid 66 is considered to be particularlyimportant; without this shielding grid, it is impossible to obtainsignificant amplification 'by secondary ionization with high voltagesapplied to the plate. When even moderate voltages are applied to theplate in triode arrangements, the iield penetrates the region betweenthe cathode and grid and produces a net reduction in output. In oneknown case, this reduction in output started at a plate Voltage level ofabout 15 volts.

With `regard to the potentials applied to the Various electrodes, itshould be noted that grid 64 must be positive with respect to thecathode 612 and have such a magnitude as to produce ionization lin thegas being detected but not in the carrier gas to any significant extent.The plate 68 must be negative with respect to the cathode 62 and shouldhave a potential with respect to the grid 64 which is significantlygreater than the ionization potential of the carrier gas. In thismanner, secondary ionization is provided. The shielding grid 66 shouldhave a potential which is between that of grid 64 and plate 68,preferably closer in potential to the accelerating grid 64 than to theplate 68, for example volts` with respect to the cathode.

The pressure within the tube 60 must -be suiciently low to permitelectron flow from cathode 62 toI grid 64. The gas pressure shouldgenerally be less than about 1 centimeter of mercury, and it isdesirable that the pressure be approximately in the range of 0.1 to lmillimeter of mercury, for most tube geometries. In particular, thepressure should be such that the mean free path of electrons emittedfrom the cathode is of the same order of magnitude or greater than thespacing between the cathode and grid.

An indirectly heated cathode is to be preferred over a lament cathode.An arrangement employing an indirectly heated cathode permits closercontrol of the potential between the cathode and grid for selectiveionization purposes. In contrast, a self-heated cathode has a signicantvoltage drop from end to end and therefore has a variablecathode-to-grid potential from end to end. For selective ionization,where diiierences of a few volts become critical, accurate fixing of thecathode-to-grid voltage is essential. The adjustable voltage sources 74,76 and 78 are shown schematically as variable batteries. It will beunderstood that suitable power supplies of the type well known in theart may be employed. The output block 80 in the plate circuit of tube 60represents any suitable amplifier or microammeter, for example. In thesystem of FIG. 1, the output device 80` would be the recorder 16 and itsassociated apparatus.

The detector 60` is capable of detecting traces of oxygen in the rangeof 1/1000 or 1;/10000 of one part per million of oxygen in a carrier gasof hydrogen. This corresponds to the detection of one part in 109 or1010 parts. With regard to the sensitivity in the detection of oxygenpresent in the source 12 of FIG. 1, the entire apparatus is capable ofdetecting approximately 1/100 or 1/1000 of one part per million ofoxygen present in the input stream. This corresponds to the detection ofone part in 108 or 109 parts. In this regard, it is considered that thelimiting factors on sensitivity are the noise, arising from impuritiesin the carrier gas, Voltage fluctuations or the like.

In the foregoing description, the apparatus of lFIG. 2 has beendisclosed as performing the function indicated by block 14 in FIG. 1. Insome cases, however, the ionization detection circuit of FIG. 2 -may beemployed without the complete apparatus as shown in FIG. l. Thus, forexample, it is applicable directly to the detection of traces of oxygenin pure nitrogen. Purified nitrogen normally contains as impuritiestraces of oxygen and argon. As both nitrogen and argon have ionizationpotentials which are significantly higher than that of oxygen, thepresence of oxygen can be detected directly with the apparatus of FIG.2. With argon having an ionization potential of 15.7 volts, that ofnitrogen being 15.5 volts, and oxygen being of only 12.5 volts, thevoltage applied to the accelerating grid 64 in FIG. 2 would beapproximately 14 volts in such a system.

As disclosed above, the detection unit 14 of FIG. 1 may be implementedby a single ionization detection tube and circuit as shown in FIG. 2.FIG. 3 represents an embodiment of the circuit of FIG. 1 in which thedetector unit 14 is implemented by the use of two detectors 14 and 14"-which lead to a dual recorder 16. In accordance with the embodiment ofFIG. 3, oxygen and argon may be detected simultaneously. These two gasesare particularly good examples as they may pass through chromatographiccolumns in approximately the same time interval. In the case of thesimultaneous analysis for both argon and oxygen, a helium carrier may beemployed. The accelerating grid potential of the detection tubes in thedetectors 14 and 14" are then set at respectively different levels.Thus, for example, in the low detector 14 the control grid potential isadjusted to a level between the 12.5 volt ionization potential of oxygenand the 15.7 Volt ionization potential of argon. The tube in detector14" has a control grid potential somewhat higher than 15.7 volts butless than the 24.5 volt ionization potential of helium. The highdetector 14" having the higher accelerating grid potential provides anoutput -signal indicating the presence of both oxygen and argon, whilethe low detector `14' is only responsive to the Vdetectors 14 and 14.

presence of oxygen. The difference in the readings between the twodetection devices indicates the quantity of argon present in themixture. The dual recorder la simultaneously registers the signals fromeach of the two In accordance' with conventional practice, the readingsmay be recorded on a moving sheet of graph paper so that the precisereading at each mornent'of time is indicated.

Various other possible combinations of gases and the vnecessary changesin the system to accommodate them will now be considered. To analyze forthe presence of nitro- Agen, it is desirable to employ a carrier gassuch as helium having an vionization potential of 24.5 volts, ratherthan a hydrogen carrier having an ionization potential of 15.6

volts, just 1/10 of a volt higher than the ionization potential ofnitrogen. In cases where it is desired to analyze for traces of benzenein a gaseous stream, a hydrogen carrier may be employed, as lbenzene hasan ionization potential of only 9.6 volts. As benzene is a heavier gas,

one or more partition columns may be employed for separation, and theabsorption column shown in FIG. 1 is not required.

In connection with the system of FIG. 1 of the drawings, a standardsource o-f oxygen containing gas may ybe provided. Under thesecircumstances, the instrument maybe calibrated periodically by insertinga sample from 'the standard source.

' It is to be understood that the above described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and `scope of the invention.

What is claimed is:

l. In 4a gas analyzing system, means for diiusion purifying -a carriergas having a high ionization potential so that it has less Ithan onepart per million impurities havving a lower ionization potential thanthe carrier gas, means `for combining said carrier gas with a compositegas sample to be analyzed which has a lower ionization potential thanthat of said carrier gas, a plurality of chromatographic separationcolumns, means lfor applying the caraccelerating grid of said tube which`is greater than the ionization potential of the `gas -being analyzedand less than the ionization potential of the carrier gas, means forapplying a potential to the plate of Said tube which is signicantlygreater'than the ionization potential of 4the carrier gas, and means forapplying a potential to the shielding grid adjacent said plate which isintermediate between the potential of said accelerating grid and thatlof said plate.

2. In a gas analyzing system, an electron tube having a gas inlet and agas outlet, means for diffusion purifying a carrier gas having a highionization potential so that it has less than one part per million ofimpurities having a lower ionization potential than the carrier gas,

`means for combining said carrier gas with a gas sample to be analyzedwhich has a lower ionization potential than that'of said carrier gas,means for supplying to said tube the carrier `gas, and the gas to beanalyzed, a cathode, an accelerating grid, a shielding grid and a platemounted within said tube, means for establishing an efective potential(between the cathode and the accelerating grid of said tube which isgreater than the ionization potential of the gas being detected but lessthan the ionization potential of the carrier gas, means lfor applying apotential to the plate `of said tube which is significantly :greaterthan the ionization potential of the carrier gas,`and means for applyinga potential to the vshielding grid adjacent said plate which isintermediate between the potential of said accelerating grid and that ofsaid plate.

3. In a gas analyzing system, an electron tube having a gas inlet and agas outlet, means for supplying to said tube a carrier gas having a highionization potential and a gas to be analyzed having an ionizationpotential which is less than that of the carrier gas, an indirectlyheated cathode, an accelerating grid, a shielding grid and a platemounted within said tube, means for establishing a potential between thecathode and the accelerating grid of said tube which is greater than theionization potential of the gas being detected but less than theionization potential ofthe carrier gas, means for applying a potentialto the plate of said tube whi-ch is significantly greater in magnitudethan the ionization potential of the carrier gas, and means for applyinga potential to the shielding grid adjacent said plate which has a valuebetween the Vpotential of said -accelerating grid and that of -saidplate.

`4. In a gas analyzing system, an electron tube having a gas inlet and agas outlet, means for purifying a carlrier gas having a 'high ionizationpotential so that it has less than one part per million of impuritieshaving a lower ionization potential than the carrier gas, means forcombining said carrier gas with a gas ysample to be analyzed which has alower ionization potential than that of said carrier gas, means forsupplying to said tube the carrier gas and the gas to be analyzed, acathode, an accelerating grid, lay shielding grid and a plate mountedwithin said tube, means for establishing an eifective p otential betweenthe cathode and the accelerating grid of said tube which is greater thanthe ionization potential olf-the gas being detected but less thany theionization potential of the carrier gas, means for applying a potentialto the plateA of -said tetrode which is signicantly greater than theionization potential of the carrier gas, and means Ifor applying apotential to the shielding grid adjacent said plate which isintermediate between the potential of said accelerating grid and that ofsaid plate.

5. In a gas analyzing system, means for diffusion puriple to be analyzedwhich has a lower ionization potential Ithan that of said carrier gas,at least one chromatographic separation column, means Afor applying thecarrier gas and the gas sample to be analyzed to said column, anelectron tube having a gas inlet and a gas outlet, means for couplingthe gas from said column to said electron tube, a cathode, anaccelerating grid, a shielding grid and a plate mounted within saidtube, means for establishing a potential between the cathode and theaccelerating grid of said tube which is greater than the ionizationpotential of the gas being analyzed and less than the ionizationpotential of the carrier gas, means for applying a potential to theplate of said tube which -is significantly greater than the ionizationpotential of the carrier gas, and means for applying a potential to theyshielding grid adjacent said plate which is intermediate between Ithepotential of said accelerating grid and that of said plate.

6 In a gas analyzing system, means for diffusion purifying a carrier gashaving a high ionization potential so that it has less than one rpartper million impurities having a lower ionization potential than thecarrier gas, means for combining said carrier gas with a composite gassample to be analyzed which has a lower ionization potential than thatof said carrier gas, va plurality of chromatographic separation columns,means for applying the carrier gas and the gas sample Ito be analyzed tosaid columns in series, an electron tube having a gas inlet and a gasoutlet, means for coupling the4 gas from said columns to said electrontube, a cathode, an accelerating grid, a shielding grid and a platemounted within said tube, means for establishing a potential between thecathode and the accelerating grid of said tube which is greater than theionization potential of the gas being detected but not significantlygreater than the ionization potential of the carrier gas, means forapplying a potential to the plate of said tube which is significantlygreater than the ionization potential of the carrier gas, and means forapplying a potential to the shielding grid adjacent said plate which isbetween the potential of said accelerating grid and that of said plate,a recorder coupled to receive output signals from said electron tube,and timing means for operating said combining means and for enablingsaid recorder at a predetermined time following the operation of saidcombining means.

7. In a gas analyzing system, an electron tube having a gas inlet and agas outlet, means for supplying to said tube a carrier gas having a highionization potential and a gas to be analyzed having an ionizationpotential which is less than that of the carrier gas, a cathode, anaccelerating grid, a shielding grid and a plate mounted within saidtube, means for establishing -a positive potential between the cathodeand the accelerating grid of said tube which is greater than theionization potential of the gas being detected but less than theionization potential of the carrier gas, means for applying a negativepotential to the plate of said -tetrode which is signicantly greaterwith respect to said accelerating grid than the ionization potential ofthe carrier gas, and means for applying a potential to the shielding.grid located between the plate and the accelerating grid which has avalue between the potential of said accelerating grid and that of saidplate.

8. In a gas analyzing system, an electron tube having a ,gas inlet and agas outlet, means for supplying to said tube a background gas includingone or more gases having high ionization potential and a gas to beanalyzed having an ionization potential which is less than that of thebackground gas, a cathode, a plate and at least two grids mounted withinsaid tube, means for establishing a positive potential between thecathode and the accelerating grid of said tube which is greater than theionization potential of the :gas being -analyzed and less than theionization potential of the background gas, means for applying anegative potential to the plate of said tetrode which is signiiicantlygreater than the ionization potenti-al of at least one of the gases ofthe background gas, and means for applying a potential to the shielding-grid adjacent said plate which is intermediate between the potential ofsaid accelerating grid and that of said plate.

9. In a gas analyzing system, means for diffusion purifying a carrier`gas having a high ionization potential so that it has less than onepart per million impurities having :a lower ionization potential thanthe carrier gas, means for combining said carrier gas with a composite,gas sample to be analyzed which has a lower ionization potential thanthat of said carrier gas, 1a plurality of chromatographic separationcolumns, means for applying the carrier gas and the gas lsample to beanalyzed to said columns in series, an electron tube having a gas inletand a gas outlet, means or coupling the gas from said columns to saidelectron tube, a cathode, an laccelerating grid, a shielding grid and aplate mounted Within said tube, means for establishing a potentialbetween the cathode and the accelerating grid of said tube which isgreater than the ionization potential of the gas being detected but lessthan the ionization potential of the carrier gas, means for applying apotential to the plate of said tube which is signicantly greater thanthe ionization potential of the carrier gas, means for applying apotential to the shielding grid adjacent said plate which isintermediate between the potential of lsaid 4accelerating grid and thatof said plate, a recorder coupled to receive output signals from saidelectron tube, and .timing means for enabling said recorder flor a timeinterval encompassing the ltime period 10 during which the gas sample tobe analyzed is leaving said columns yand entering said electron tube.

l0. In a gas analyzing system, means for -diiusion purifying a carriergas having a high ionization potential so that it has less than one partper million of impurities having a lower ionization potential than thecarrier gas,

vmeans for combining said carrier gas with a composite gas sample to beanalyzed which has a lower ionization potential than that of sai-d`carrier gas, a plurality of chromatographic separation columns, one ofsaid columns being a partition column and another of said columns -beingan adsorption column, means for applying the carrier gas and the gassample to be analyzed to said columns in series, an electron tube havinga gas inlet and a gas outlet, means for ycoupling the gas from saidcolumns to -said electron tube, a cathode, an accelerating grid, ashielding grid and a plate mounted with said tube, means forestablishing a potential between the cathode and the accelerating gridof said tube which is greater than the ionization potential of the gasbeing analyzed and less than the ionization potential of the carriergas, means for applying a potential to the plate of said tube which issignificantly greater than the ionization potential of the carrier gas,and means for applying a potential to the shielding grid adjacent saidplate which is intermediate between the potential of said acceleratinggrid and that of said plate.

1l. In a gas analysis system, at least one chroma- `tographic column,means for applying a gas sample to said column including two gases whichhave approximately the same rate of migration through said column, atleast two ionization detectors coupled to the output of said column,means for establishing an ionization potential in one yof said detectorswhich is between the ionization potentials of said two gases, means forestablishing an ionization potential in the other of said detectorswhich is above the ionization potential of both of said gases, eachldetect-or including a plate for collecting ions to produce outputsignals, and means for recording the output signals from both of thedetectors.

l2. -In a gas analyzing system, an electron tube having .a gas inlet anda gas outlet, means for supplying to said tube a carrier gas having ahigh ionization potential and a gas to be analyzed having an ionizationpotential which is less than that of the carrier gas, an indirectlyheated cathode, an accelerating grid and a plate mounted within saidtube, means for establishing a positive potential between the indirectlyheated cathode and the accelerating grid of said tube which is greaterthan the ionization potential of the gas being detected but less thanthe ionization potential of the carrier gas, and means for applying apotential to the plate of said tube which is negative with respect tothe potential of said grid.

13. In a gas analysis system, at least two ionization detectors, meansfor applying a gas sample including two gases which have differentionization potentials to both of said detectors, means for establishingan ionization potential in one of said detectors which is between theionization potential of said two gases, means for establishing anionization potential in the other of said detectors which is above theionization potential of both of :said gases, each detector including aplate for collecting ions to produce 4output signals, and means forrecording the output signals from both of the detectors.

14. In a gas analysis system, at least on chromatographic column, acarrier gas having a relatively high ionization potential, a gas sampleincluding two gases which have approximately the same rate of migrationthrough said column and which have lower ionization p-otentials thansaid carrier gas, means for combining said sample with said carrier gasand for applying the combined gases to said column, at least twoionization detectors lcoupled to the output of said column, means forestablishing an ionization potential in one of said detectors which isbetween the ionization potentials of said two sampled gases, means forestablishing an ionization potential in the other of said detectorswhich is above the ionizationpotential of both of said sampled gases,each detector including a plate for collecting ions Vto produce outputsignals, and means for recording the outputl signals from both ofthedetectors.

15. In a gas analysis system, at least one chromaltographic column, acarrier gas having a relatively high ionization potential, a gas sampleincluding two gases which have approximately the same rate of migrationthrough said column and which have lower ionization potentials than saidycarrier gas, means `for combining said sample with said carrier gas andfor applying the combined gases to said column, at least two ionizationdetectors coupled to the output of said column, means for establishinglan ionization potential in one of said detectors whichvis between theionization potentials of said Atwo sampled gases, means for establishingan ionization potential in the other of said detectors which is abovethe ionization potential of both of said sampled gases, each detectorincluding a plate for collecting ions to produce output signals, Iandmeans for recording the output signals from both of said detectors.

16. In a gas analysis system, at least one chroma- Ytographieh column,means for applying a gas sample yto said column including two gaseswhich have different ionization potentials, at least two ionization`detectors coupled to the output of said column, means for establishingan ionization potential in one 4of said detect-ors `which is between theionization potentials of said two gases, means for establishing anionization potential in -means for recording the output signals fromboth of the detectors.

References Cited in the tile of this patent UNITED STATES PATENTS2,579,352 white Dec. 18, 1951 2,761,976 Obermaier Sept. 4, 1956 202,770,772 Foulkes Nov. 13, 1956 FOREIGN PATENTS 805,034` Great BritainNov.16, 1958

1. IN A GAS ANALYZING SYSTEM, MEANS FOR DIFFUSING PURIFYING A CARRIERGAS HAVING A HIGH IONIZATION POTENTIAL SO THAT IT HAS LESS THAN ONE PARTPER MILLION CARRIER GAS, MEANS ING A LOWER IONIZATION POTENTIAL THAN THECARRIER GAS, MEANS FOR COMBINING SAID CARRIER GAS WITH A COMPOSITE GASSAMPLE TO BE ANALYZED WHICH HAS A LOWER IONIZATION POTENTIAL THAN THATOF SAID CARRIER GAS, A PLURALITY OF CHROMATOGRAPHIC SEPARATION COLUMS,MEANS FOR APPLYING THE CARRIER GAS AND THE GAS SMAPLE TO BE ANALYZED TOSAID COLUMNS IN SERIES, AN ELECTRON TUBE HAVING A GAS INLET AND A GASOUTLET, MEANS FOR COUPLING THE GAS FROM SAID COLUMNS TO SAID ELECTRONTUBE, A CATHODE, AN ACCELERATING GRID, A